Minimizing ground potential differences
Low-level, single-ended voltage measurements (<200 mV) are sensitive to ground potential fluctuation due to changing return currents from 
) from power grounds (G). For more information on data logger grounds, see Grounds. To take advantage of this design, observe the following rules:
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Connect grounds associated with
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Connect excitation grounds to the nearest
terminal on the same terminal block. -
Connect the low side of single-ended sensors to the nearest
terminal on the same terminal block. -
Connect shield wires to the
terminal nearest the terminals to which the sensor signal wires are connected.
If offset problems occur because of shield or ground wires with large current flow, tying the problem wires into terminals next to terminals configured for excitation and pulse-count should help. Problem wires can also be tied directly to the ground lug to minimize induced single-ended offset voltages.
Ground potential differences
Because a single-ended measurement is referenced to data logger ground, any difference in ground potential between the sensor and the data logger will result in a measurement error. Differential measurements MUST be used when the input ground is known to be at a different ground potential from data logger ground.
Ground potential differences are a common problem when measuring full-bridge sensors (strain gages, pressure transducers, etc), and when measuring thermocouples in soil.
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Soil Temperature Thermocouple: If the measuring junction of a thermocouple is not insulated when in soil or water, and the potential of earth ground is, for example, 1 mV greater at the sensor than at the point where the data logger is grounded, the measured voltage will be 1 mV greater than the thermocouple output. With a Type T (copper-constantan) thermocouple, 1 mV equates to approximately 25 °C measurement error.
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External Signal Conditioner: External instruments with integrated signal conditioners, such as an infrared gas analyzer (IRGA), are frequently used to make measurements and send analog information to the data logger. These instruments are often powered by the same VAC-line source as the data logger. Despite being tied to the same ground, differences in current drain and wire resistance result in different ground potentials at the two instruments. For this reason, a differential measurement should be made on the analog output from the external signal conditioner.
For additional information, see Minimizing offset voltages.